AK-pattern firearms, while known for their simplicity and reliability, are not known for precision. As used herein, “AK-pattern” firearm is mean to include firearms of the Avtomat Kalashnikov or AK family, including the AK-47, AKM, AK-103, AK-74, AKS and others built on an AK type receiver, such as the Saiga family of rifles and shotguns. While simplicity of design and manufacturing are not mutually exclusive of precision, many owners of AK-pattern firearms made without precision often wish to upgrade certain features and aspects of the firearm with aftermarket products, parts, or accessories. A common “upgrade” is to install a new trigger mechanism, which generally includes a trigger member, disconnector, and hammer with associated springs and pivot pins.
In a standard or OEM (original equipment manufacturer) configuration, the hammer and trigger of an AK-pattern firearm are mounted to pivot on assembly pins that extend through openings in opposite side walls of the receiver. Most commonly, the receiver is formed from a flat sheet of metal that is stamped to form its finished shape. Openings in the receiver side walls that receive assembly pins may be punched or drilled in the sheet of metal prior to stamping. While it is intended that the finished location of these openings be precisely aligned with one another and provide axes of rotation transverse to the receiver and that are exactly parallel to one another, in practice, they may not be. The sometimes imprecise location of these openings makes it difficult to retrofit the firearm with a precision trigger mechanism.
Others have addressed this shortcoming by producing “drop-in” trigger or fire control modules that are held together as a unit by a frame or housing. However, because the final assembly pins that hold the module in place within the receiver extend through and form the axis of rotation for the trigger member and hammer, imprecision in the original receiver can result in distortion of the module causing misalignment of critical interfaces, such as the sear and/or disconnector to the hammer. Some manufacturers of these “drop-in” trigger mechanism units even expect such misalignment and instruct the installer to file or otherwise alter these critical interfaces to make the replacement trigger mechanism functional. While such custom fitting may be within the skill of a gunsmith, many end users who do their own customization by using a “drop-in” replacement trigger unit are not so skilled and expect the replacement mechanism to function properly and easily without modification of critical parts.
“Drop-in” replacement trigger modules designed for other types of firearms, such as the AR15, do not face these problems. The receivers of AR15-pattern firearms are generally milled with relative precision from a billet of material or a forging blank, not a stamped sheet. In addition to the relatively precise placement of openings for the assembly/pivot pins, the interior dimensions of the receiver's trigger group receiving area, including the transverse width between side walls, is typically held to close tolerances. The design of OEM parts for AR15-pattern firearms expect this level of precision, not commonly found in AK-pattern firearms, so the same can be expected from designers and manufacturers of “drop-in” replacement trigger modules for the AR15.